A direct measurement of the tilting rate of the Milky Way disc

RESUMO

The Milky Way disc is typically described, at lowest order, as a flattened rotating stellar component with a fixed spin axis. However, interactions and mergers with smaller galaxies are expected to have an impact on the disc properties. Such interactions can leave long-lasting signatures within galactic discs, including warps and corrugation waves that have already been detected in the Milky Way, but also a slow reorientation of the disc known as tilting, which has never been directly measured up to now. Here, using the vertical proper motions of 110 million stars from the Gaia DR3 catalogue, we present the first-ever direct measurement of the tilting rate of our Galaxy's disc. The detected signal of 14 +/- 2 deg/Gyr around the Sun-Galactic Centre axis is statistically significant and is consistent with typical values found in cosmological simulations. By comparing our measurements with high-resolution tailored simulations, we infer that the observed tilting rate is likely the long-term consequence of a major accretion event that occurred around 8-10 billion years ago. The signal is consistent with the merger of a massive satellite galaxy on a retrograde orbit with a ratio of about 1:4, probably associated with the Gaia-Enceladus/Sausage event. These results provide independent evidence for a key episode in the Milky Way's formation history and introduce a new way to reconstruct the assembly histories of disc galaxies from their present-day motions.